Q.55 When a plant is infected by a pathogen at one site, the distal parts of the plant and neighboring
plants develop increased resistance to subsequent pathogen attack. Which of the following molecules
mediates this long-distance signal?
(A) Nitric oxide
(B) Ethylene
(C) Jasmonic acid and its derivatives
(D) Salicylic acid and its derivatives

Plants activate systemic acquired resistance (SAR) when infected by pathogens at one site, priming distal parts and neighboring plants against further attacks. This phenomenon involves specific signaling molecules traveling long distances. The correct answer is (D) Salicylic acid and its derivatives, as they serve as the primary mobile signal for SAR.​

Correct Answer Explanation

Salicylic acid (SA) and its derivatives, like methyl salicylate, accumulate in infected tissues and translocate via phloem or as a volatile to uninfected distal parts, triggering defense gene expression such as PR proteins. This establishes broad-spectrum resistance lasting days to weeks, independent of jasmonic acid or ethylene pathways. Studies confirm SA’s role in SAR, distinguishing it from local hypersensitive responses.​

Option Analysis

• (A) Nitric oxide: Acts as a local reactive signal in early pathogen recognition, promoting cell death and defense bursts at infection sites, but lacks evidence for long-distance transport in SAR.​

• (B) Ethylene: Functions in local stress responses, wounding, and necrotroph defense by inducing PR genes or senescence, yet fails to mediate SAR’s distal priming.​

• (C) Jasmonic acid and its derivatives: Key for induced systemic resistance (ISR) against herbivores or necrotrophs via root microbes, operating locally or in JA/ET synergy, not SAR’s biotrophic pathogen focus.​

SEO Keywords

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This mechanism highlights SA’s pivotal role in plant immunity, vital for agriculture and biotech applications like priming crops against diseases.​